Effects of auditory stimuli correlated with different probabilities of water delivery in a limited-hold temporal schedule

Four rats were exposed to two different tone frequencies, each tone being correlated with independent probabilities of water delivery in a temporally defined schedule. The schedule consisted of a 60-s T cycle, with 30-s t(D) and t(Delta) succesive subcycles. t(D) and t(Delta) were assigned complementary probabilities of water delivery (1.0-0, 0.75-0.25, 0.5-0.5 and 1.0-0) in succesive phases. Each of the first three experimental phases was followed by five consecutive probe sessions using the same probability values, and in which water was delivered independently of responding at the end of each subcycle. Three additional rats were exposed to the same conditions, except that only one tone was used in t(D) and t(Delta). The highest frequencies of responding were observed in two of the rats exposed to differential tone frequencies, an effect that was correlated with lower percentages of lost water deliveries, shorter response latencies, and less variability in the intervals between successive water presentations. In the last phase in which probabilities of water delivery in t(D) and t(Delta) were 1.0 and 0, respectively, response frequency was always higher in t(Delta). These results suggest that the number of water deliveries obtained in the early cycles of every session were responsible for the total number of responses and its correlated effects.